(1) Field of the Invention
This invention relates generally to electrochemical noise and more particularly to a method of using electrochemical noise for determining pit initiation rates for an aluminum alloy.
(2) Background of the Invention
The introduction of high-strength aluminum alloys during the past 40 years has solved many of the problems associated with reducing the weight of components formerly manufactured from ferrous alloys. A case in point is the use of aluminum alloy 6061 having the following composition.
______________________________________ Magnesium 1.0 Silicon 0.6 Copper 0.25 Chromium 0.25 Aluminum 97.9 ______________________________________
Unfortunately, while these low density alloys have improved the strength-to-weight ratio desirable for many marine engineering applications, they also have brought with them their own unique set of corrosion problems. In particular, pitting corrosion in the marine environment is devastating for these alloys.
Corrosion resistance of an aluminum alloy results from the protective and tightly adherent aluminum oxide film which naturally forms on its surface. If this 50-100 angstroms thick film remains intact in an environment, corrosive breakdown cannot occur. Obvious sites for film breakdown are scratches or sharp discontinuities (where the film is mechanically stressed) and in regions where reactive ions speed up dissolution of the film. While the oxide film tends to repair itself if broken through, the environment or a pitting reaction itself may prevent it from reforming. Pitting corrosion of aluminum 6061, like that of other metal alloys, is associated with the flow of electric current between anodic and cathodic regions. The general mechanism by which this occurs is through the development of a corrosion cell or pit where oxidation and reduction reactions occur and are self-sustaining (autocatalytic).
There is depicted FIG. 1 in a cross-sectional diagrammatic illustration of a well-established pitting cell 15. Pitting cell 15 is shown in an aluminum alloy 10 that is exposed to an aqueous chloride environment or solution 100. Aluminum alloy 10, shown only in section, is comprised of an aluminum alloy base 11 and a corrosive-resistant aluminum oxide (Al.sub.2 O.sub.3) film 13. Film 13 is naturally formed and is tightly adhered to the surface 11a of base 11. The surface 13a of film 13 is exposed to the aqueous chloride environment 100. A crust 14 of Al(OH).sub.3 precipitate forming over the pit 15 restricts solution 100 from entering the interior of pit 15. Accordingly, solution 100 enters the pit 15 only through a pore, designated generally by 14a, in the crust 14. The growth of the pit 15 involves the interaction of the aluminum base 11 directly with the solution 100 within the pit 15.
Note that exposed noble precipitates 17 can support oxygen reduction as an inert cathodic surface and thus provide cathodic protection for the areas immediately surrounding them. Noble precipitates 17 are local "islands" of precipitate alloy formed during the alloy solidification process. They are "noble" from the standpoint of being more corrosion resistant than the surrounding homogeneous alloy structure. They serve as discontinuities in the atomic lattice structure which inhibit the movement of lattice dislocations in order to strengthen the alloy 10. Pitting thus confines itself to specific areas of the alloy surface while adjacent surfaces remain virtually unaffected. This accounts for the non-uniform nature of pitting corrosion. Ultimately, it is the depletion of oxygen within the restricted confines of the pit 15 that allows the reaction to continue and precludes the formation of oxide film 13. This is evidenced by the predominance of other reduction reactions within the pit 15.
Pit conditions are transient as concentrations of ions necessary to support pitting reactions can be swept away by local solution currents and mixing. This in fact has been observed to occur, with the passive film 13 then reforming over a former pit location. The transient nature of pitting corrosion cells thus offers insight into the origin of electrochemical noise (ECN) signals. The pitting attack on the aluminum base 11 is observed, not as a steady and smooth reaction, but as an erratic and discontinuous process where the electric potential across the film 13 rises and falls as the reaction rate varies. This is evidenced not only by potentiodynamic cycling (that is, bursts of electrochemical noise) but also by acoustic emissions that may be observed during the pitting of aluminum. Currently, however, there is no method of correlating the electrochemical noise signals generated by the pitting process to pit initiation rates.